Polarized lenses have been used in sunglasses and other types of eyewear for a number of years and have proved to be of particular value in reducing glare and other potentially irritating effects for the wearer. The conventional fabrication method for polarizer material uses a layering in which a sheet of polarized film material is sandwiched or laminated between outer layers of a glass or plastic. The polarizer material itself is a thin sheet of polymer that has its molecules aligned or oriented, such as by stretching in one direction. Subsequent treatment, such as with dyes and lamination, forms a single axis polarizer sheet that can then be used for sunglasses and other eyewear.
It is known that glare is predominantly horizontally polarized light, since horizontally polarized light reflects much more strongly off a flat surface than does vertically polarized light. Therefore, in order to minimize glare and reduce excessive outdoor brightness, polarizing sunglasses are generally designed to block a higher percentage of horizontally polarized light and transmit a portion of vertically polarized light. Referring to FIG. 1, incoming light from the sun or other bright source impinges on a horizontal object 10 having a top specular surface 11. Surface 11 could be, for example, a thin layer of water covering object 10. Light reflecting off specular surface 11 is glare. The incoming sunlight or other light is unpolarized or randomly polarized light. Light reflected from surface 11 is more highly s-polarized, as represented schematically in FIG. 1; light scattered from object 10 just under top surface 11 is commonly unpolarized. Well-designed polarized eyewear 12, with its polarization transmission axis well-aligned, generally blocks s-polarized light and transmits p-polarized light. For conventional polarized sunglasses, for example, a vertical transmission axis, shown as V in FIG. 1, is most appropriate.
Having a suitable polarization axis, then, helps polarized sunglasses to reduce glare and other effects. However, in practice, maintaining the best polarization axis for viewing through polarized eyewear can be difficult. Certainly, some amount of viewer head movement is inevitable and can be difficult to compensate while maintaining desired polarization. But a more significant problem for maintaining the preferred polarization axis relates to lens shape and sunglass design. Conventional polarizers do not easily adapt to “wraparound” lens shapes or to more highly curved spherical and “sun-lens” designs. Designs that require more than minimal bending and curving of the polarized plastic and designs with curvature about more than one axis tend to cause inconsistent and non-uniform stretching of the polarizer material and can easily warp and degrade the performance of the polarizer film. Further, fabrication methods such as thermoforming, commonly used to bend and adapt plastic materials to conform to a given shape or curvature, can induce stress into the polarizer material, thereby deforming any parallel molecular pattern that was provided for obtaining polarization in the first place. A stretched pattern of molecules can be warped out of their intended alignment by any type of shaping operation, making the corresponding polarizer less effective, compromising the extinction ratio in an irregular fashion over the lens surface, leading to an inconsistent polarization pattern across the field of view, and even introducing image aberrations and artifacts into the lens. In extreme cases, this effect can be distracting, can lead to image distortion, and might even be hazardous for the viewer.
Yet another problem with conventional polarizer fabrication relates to waste. Because of the nature of handling thin plastic substrates, stretching of the polarizer film itself is a relatively imprecise process that is difficult to control, and the end-product is often subject to non-uniformity. It has proven to be difficult to provide the uniform stress pattern needed for maintaining a polarization axis and relative success or failure may not be known until after fabrication has been completed. This, in turn, leads to reduced yields.
Because of these problems, polarized lens material, available only for designs with limited curvatures, more uniform cylindrical curvatures, and simpler shapes, is often difficult to adapt to eyewear design and consumer tastes. Difficulties in controlling the polarization axis for any particular lens shape constrain the ability of lens designers to take advantages of inherent strengths and benefits of polarizers for the eyeglass wearer.
Photochromic materials that become increasingly opaque upon exposure to light have been used for providing sunglasses that adapt their opacity to the ambient light level. Conventional photochromic sunglasses exhibit this behavior according to the relative levels of incident ultraviolet (UV) light, so that the same pair of sunglasses can be worn in bright sunlight and in shaded or indoor lighting conditions. Under bright sunlight, UV levels are high and the photochromic coatings become proportionately more opaque. When the same glasses are worn indoors, the photochromic effect is reversed, with the lens coatings becoming proportionately more transparent.
While the photochromic effect has been found useful for blocking suitable amounts of light under various conditions, however, there are shortcomings to conventional designs. One drawback relates to relatively slow response times, making photochromic behavior less desirable where brightness transitions are sudden and extreme, or where light is intense over only a small portion of the field. Another problem relates to poor spectral response of many photochromic materials for light outside the UV range. Because of these and related problems, there is limited applicability of photochromic benefits to industrial, transportation, or defense safety using conventional design approaches.
Thus, it can be appreciated that there would be advantages for polarized eyewear having variable opacity and capable of responding more quickly to a range of possible incident light conditions.